Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare destructiveHint=true and idempotentHint=true; the description is consistent with both and adds the concrete meaning of the destructive behavior (the previous primary is unset). It also discloses output-side obligations (attribution footer handling, data-accuracy contract), though that text is generic boilerplate rather than tool-specific behavior.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The first two sentences are tight and front-loaded with the core behavior. However, roughly half the description is generic attribution-footer and data-accuracy boilerplate that is not specific to this tool and dilutes the signal.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a single-parameter mutation tool with no output schema, the description covers the effect, the cardinality constraint, and the downstream default-target semantics. What is missing is minimal (e.g., what happens on an invalid or unregistered channel_id).

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100% and the single channel_id parameter is already documented there. The description only reinforces that the target must be an already-registered Brand Account, adding modest meaning beyond the schema's own wording.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource ('Change which already-registered Brand Account is the user's primary') and clearly separates this from sibling tools like add_my_youtube_brand_channel and remove_my_youtube_brand_channel. An agent can identify it without opening the schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Explains the practical consequence of the action: the primary brand is the default target for per-channel my_youtube_* tools when no channel_id is supplied, and states the cardinality rule (at most one primary; setting a new one unsets the old). It does not explicitly name prerequisites such as listing channels first or registering a brand before setting it primary.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.